The present invention is directed to testing flat-panel displays and in particular to testing the matrices of thin-film transistors that are employed in active-matrix liquid-crystal display panels.
An active-matrix liquid-crystal display is a flat-panel device that provides a screen divided into pixels, each of which, in a color version, contains three liquid-crystal devices disposed between a backlighting device and respective red, green, and blue filters. By varying the potential across the liquid-crystal devices (LCDs), one can change their light transmittances and thus form an image on the screen by selectively driving the LCDs.
The pixels are organized in rows and columns in the conventional manner. In an active-type LCD display, a separate transistor drives each color in each pixel. All the LCDs have one of their electrodes connected to a common potential, and each has its other electrode connected to the source or drain of its corresponding transistor. The other channel terminal (source or drain) of the transistor is connected to a data line, which is similarly connected to all of the transistors for the same color in the same column. The transistor gate is connected to a select line, which is similarly connected to all transistors in the same row.
To drive the display, one places an enabling voltage on the select line of the row to be scanned and applies drive voltages indicative of the desired brightnesses to the various column lines individually. Although those drive voltages are applied to the transistors in all of the rows, the light transmittances of the LCDs in only the selected row are affected, because only those transistors have been turned on so as to couple the drive voltages to the transistors.
Flat-panel displays are among the most difficult of electronic devices to fabricate. The drive matrix typically includes over 100,000 transistors, and the LCD panel has an equal number of LCDs. All of those devices must operate properly, or at least almost all of them must, and they must be appropriately aligned with their corresponding LCDs. For these reasons, many of the steps in the process typically result in very low yields, and it is important to test component parts at the early stages of the process so as to avoid incorporating defective parts into later processes and thus wasting time, material, and money. In particular, it is important to test the drive matrix before it is connected to the LCD layer.
However, testing the matrix before it is assembled into the completed part presents significant problems because the matrix lends itself neither to functional testing nor to in-circuit testing. In-circuit testing, i.e., probing internal nodes so as to test the functions of the individual internal devices--in this case the individual transistor drives--is difficult; the small sizes of the transistors makes probing their individual terminals with conventional probes virtually impossible, while the use of more-exotic devices such as scanning electron microscopes to determine internal node voltages can be prohibitively expensive.
Because of the relatively low number of readily accessible terminals, it might initially seem preferable to employ functional testing, in which only the overall result of the complete circuit's operation is verified by stimulating the device from readily accessible contact points and observing the external behavior. Unfortunately, the matrix's overall function is to vary the transparencies of LCD elements to which the matrix has not yet been connected, so there is little external behavior to observe.
Testing drive matrices thus does not lend itself to traditional test approaches.